Foam absorber

ABSTRACT

A foam absorber having electromagnetic energy attenuation characteristics comprising an open cell reticulated polyurethane foam impregnated with a conductive ink applied to a surface of the foam substrate. The conductive ink is applied as by spraying to a surface of the foam, coating such surface and gradiently loading the foam with the ink by penetration of the ink in gradually decreasing amount into the foam to a final depth therein. The conductive ink is comprised of an epoxy resin carrier and a combination of carbon and a metal such as silver, copper or nickel, preferably silver. The ink is applied as a curable mixture containing a solvent, to the foam substrate, the solvent is evaporated following coating and penetration of the ink into the foam, and the epoxy carrier of the ink in the foam substrate is cured.

This is a divisional of application Ser. No. 749,993, filed Aug. 26,1991.

BACKGROUND OF THE INVENTION

This invention relates to attenuation of electromagnetic energy, and isparticularly directed to a foam absorber which is effective forattenuating electromagnetic energy especially for radar attenuation,over a broad range of frequencies.

The use of an open-cell foam substrate having a filler forming aconductive loading gradient as a light-weight broadband microwaveabsorber is known. However, when such prior art absorbers are flexed inmany instances the elasticity of the foam is reduced to a point whereupon continuing flexure, the loading material fractures and theconductive gradient is adversely affected. This is due in large measureto the fact that the conductive gradient loading material has adifferent flexibility from that of the foam and does not bend with thefoam substrate upon flexure thereof.

In addition, prior art foam absorbers often suffer from the problem ofimproper distribution of the conductive ink filler material in thegradiently loaded substrate.

Accordingly, one object of the invention is the provision of an improvedfoam absorber for attenuation of electromagnetic energy.

Another object is the provision of an open cell foam absorber gradientlyloaded with a conductive material which has a flexibility which closelymatches the flexibility of the foam substrate.

A still further object is to provide a foam absorber having a dielectricgradient which attenuates electromagnetic energy or radar over a broadrange of frequencies.

Yet another object is the provision of a foam absorber for attenuationof electromagnetic energy, having improved distribution of thegradiently loaded dielectric material within the foam substrate.

Still another object is to provide procedure for producing the improvedfoam absorber of the invention.

SUMMARY OF THE INVENTION

The above objects are achieved according to the invention by theprovision of a foam material having electromagnetic energy attenuationcharacteristics which comprises an open cell reticulated polyurethanefoam substrate impregnated with a specific conductive ink applied to asurface of the foam substrate and gradiently loading the foam substratefrom such surface with the ink to a desired depth within the substrateto provide a dielectric gradient therein. The ink is comprised of anepoxy resin carrier containing a conductive material consistingessentially of a combination of carbon and a metal such as silver,copper or nickel, particularly silver. Using this technique, adielectric gradient is achieved which attenuates electromagnetic energyover a broad range of frequencies.

If desired, the foam substrate can be initially treated with a fireretardent material. Further, various skin materials such as cloth can bebonded as by adhesive to the treated surface of the foam substrate as anenvironment barrier.

The foam absorber of the invention can be readily flexed over a longperiod of use without damage to the flexible loading material. Further,the foam absorber of the invention can be provided with a more uniformdistribution of the gradiently loaded ink from the surface to which theconductive ink is initially applied to the final depth of penetration ofthe ink within the foam substrate.

The foam product of the invention can be made thinner and lighter thanconventional foam absorbers, and is more durable and reproducible thanexisting reticulated foam absorbers. The foam absorber product of theinvention retains efficient performance characteristics upon mechanicalcycling, and is durable and has increased chemical resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the process for applying a conductive ink onto asurface of a reticulated foam to provide a gradient loading onto thefoam substrate;

FIG. 2 illustrates the resulting foam absorber of the invention,produced by the process illustrated in FIG. 1; and

FIG. 3 is a plot illustrating the dielectric gradient profile for twofoam substrates treated according to the invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawing, a conductive ink according to theinvention, and described more fully below, in the form of a liquidmedium, is sprayed, as indicated at 10, from a spray gun 12 onto thefront surface 14 of a reticulated foam substrate 16.

The reticulated foam substrate 16 employed according to the invention isa light weight, flexible, reticulated polyurethane foam formed byexploding bubbles of the foam material, using large open cells 18 withina skeleton structure of fibers, similar to a fiber mat. The reticulatedcell foam substrate functions as a filtering medium for the conductiveink which is sprayed onto the front surface 14 of the foam substrate.The foam substrate can be a polyether polyurethane foam or a polyesterpolyurethane foam, preferably the latter. The porosity of thereticulated polyurethane foam substrate generally ranges from about 3 toabout 20 ppi (pores per inch).

The ink system applied to the open cell polyurethane foam substrate isin the form of a conductive paint comprised of a carrier in a solventand loaded with a conductive medium. The resin carrier employed is aflexible epoxy which upon curing has essentially the same flexibilitycharacteristics as the polyurethane foam substrate, that is theflexibililty of the cured epoxy closely matches the flexibility of thepolyurethane foam Other resin carriers for the ink such as acrylics andpolychloroprene rubber have been tried but have not been found to havethe advantageous flexibililty properties of the flexible epoxies.

The solvent medium for the ink composition can be any suitable organicsolvent such as methyl ethyl ketone (MEK), toluene, and a "Cellosolve"such as butyl Cellosolve acetate, a trademark of Union CarbideCorporation for ethylene glycol monobutyl ether acetate.

The conductive material or filler of the ink composition is acombination of carbon, e.g. graphite powder, and a metal which can besilver, copper or nickel. It has been found that silver is the preferredmetal for this purpose. The range of proportions of carbon to metal inthe ink system can range from about 0.5 to about 2 parts of carbon, toabout 1 part of the metal, by weight. It has been found that a ratio ofcarbon to metal, particlularly silver, of about 1.7 to about 1 part, byweight, is particularly preferred. The carbon component decreases theconductivity of the ink and confers dielectric properties on the ink,the greater the amount of carbon, the greater the dielectriccharacteristics of the ink composition.

The ink composition or filler material, that is the above combination ofcarbon and metal, particularly silver, both in particlate or powderform, is blended into the solvent medium containing the epoxy resincarrier. The ink system containing the carbon and metal particles andepoxy resin in homogeneous suspension can contain a total of about 4 toabout 35% solids, e.g. about 20% of a combination of carbon, metal, e.g.silver, and epoxy resin, by weight.

As previously noted, the ink system is preferably applied like a spraypaint to the reticulated polyurethane foam substrate with an atomizingspray gun. The ink spray can be applied in one or several passes to thefoam substrate to increase the total amount of ink composition coated onand impregnated into the foam.

The ink system or paint thus applied to the foam substrate 16, as shownin FIG. 1, forms a surface coating on the front surface 14 of the foamsubstrate. There is a minor solvation of the epoxy carrier resin in thesubstrate but such solvation is negligible. As the ink is continued tobe applied to the foam substrate, the paint penetrates and filtersthrough the foam in a gradually decreasing amount from the coatedsurface 14 of the substrate, to a final depth within the foam. Theconductive ink can penetrate or impregnate into the foam substrate to athickness ranging from about 0.1 to about 4 inches. The amount ordensity of the conductive ink becomes gradually less as the inkpenetrates deeper into the foam substrate, gradiently loading the foamsubstrate in gradually decreasing amount from the coated surface,providing a dielectric gradient from the coated surface to a final depthin the foam.

The epoxy resin carrier is then cured by curing agents which are presentin the solvent solution of the epoxy resin carrier, either at normal orelevated temperature. Upon curing, a foam absorber having a dielectricgradient layer of the aforementioned thickness, and which attenuateselectromagnetic energy over a broad range of frequencies, is achieved.The attentuation range is dependent upon the substrate thickness.

The employment of a flexible epoxy as carrier for the ink results in acured epoxy carrier which has flexibility characteristics closelymatching those of the polyurethane foam substrate, which avoids crackingor disintegration during compression and expansion of the foam substratein use. In addition, a uniform distribution of conductive ink isachieved from the surface of the foam to which the ink is initiallyapplied and at each layer of depth down to the lowest depth ofpenetration of the ink within the foam substrate.

The foam absorber of the invention is illustrated at 20 in FIG. 2,wherein numeral 22 is the foam substrate and 24 is the ink fillerproviding the dielectric gradient within the substrate which isdielectrically graded from the coated or black side 26 of the foamsubstrate, with the ink filler filtering through the foam from thecoated side 26, in gradually decreasing amount down to the lowest depthof penetration indicated by dotted line 28.

If desired, to improve the foam absorber performance, as previouslynoted, several passes or applications of the conductive ink can beapplied to the foam substrate. Such additional applications or passescan be accomplished using the same ink system, or a different inkformulation, if desired.

The foam absorber of the invention is designed to absorb as much of theelectromagnetic energy as possible and at the lowest frequency possible,both in band width and in magnitude. Performances have been obtainedemploying the foam absorber of the invention containing an ink systemformed of a combination of carbon and silver, on the order of 20 dB from2.5 to 18 GHz, and in excess of 30 dB from 4 to 18 GHz.

If desired, the polyurethane foam substrate can be treated with a fireretardent material such as suitable phosphates or antimony trioxide,prior to treatment with the ink system.

Further, the volatile organic content of the ink system can be reducedfor environmental concerns, by employing an aqueous ink system.

To protect the foam absorber from environmental conditions, such asprotection from water and sunlight, a variety of skinning materials canbe applied to the coated surface of the foam absorber. Thus, cloth,fabric sheet or plastic can be bonded, as by a suitable adhesive, or byultrasonics, to the ink coated surface at 26 of the foam absorber 20 inFIG. 2, and/or to the opposite surface 30 of the foam absorber.

The foam absorber of the invention can be employed for any applicationto attenuate electromagnetic or radar energy, e.g. in aircraftcomponents of aircraft where it is desired to avoid radar detection, orin applications for reducing background noise levels, e.g. from anantenna.

The following are examples of practice of the invention:

EXAMPLE 1

A spray material in the form of a conductive ink is prepared having thefollowing composition:

    ______________________________________                                        Composition A                                                                 Components          Amounts                                                   ______________________________________                                        CT-5079-4           150     grams                                             CT-5030             37.5    grams                                             MEK                 2,000   ml                                                toluene             1,750   ml                                                B-97 curing agent   4.4     grams                                             ______________________________________                                    

CT-5079-4 is a composition containing 25-30 wt. percent carbon, theremainder being epoxy resin and butyl Cellosolve acetate as solvent, andmarketed by Emerson & Cuming of Woburn, Mass.

CT-5030 is a composition containing 50-70 wt. percent silver, theremainder being epoxy resin and butyl Cellosolve acetate, also marketedby Emerson and Cuming.

B-97 curing agent is also marketed by Emerson and Cuming, and isbelieved to be an amidazole.

The total solids content of Composition A is about 5% by weight.

Composition A was homogenized in a colloid mill. The resulting inkcomposition was sprayed on the front side of a 13"×13" square piece of10 ppi polyester polyurethane reticulated open cell foam having athickness of about 1.5", as illustrated in FIG. 1 of the drawing. Thespray nozzle was spaced about 6-8" from the front surface of the foamand formed a spray having about a 4" fan width. The spray was applied ata fluid pressure of about 15 psi.

The resulting foam coated and penetrated with the conductive inkcomposition was dried to remove solvent, and then placed in an oven andheated at about 200°-250° C. for about one half hour to cure the epoxyresin carrier of the conductive ink filler. The cured foam having aconductive gradient extending from the coated front side of the foaminternally thereof to a final depth within the foam body, is asillustrated in FIG. 2.

When such cured foam material was tested for electromagnetic energyattenuation, the attenuation was 20 dB at 8 to 18 GHz.

EXAMPLE 2

The procedure of Example 1 is followed except that the foam is firsttreated with antimony trioxide to render it fire retardant.

Substantially the same electromagnetic energy attenuation results as inExample 1 are obtained.

EXAMPLE 3

The procedure of Example 1 is followed except that after preparation ofthe cured foam material, containing the gradiently loaded ink, a Gortexfabric sheet, a trademarked material of W. L. Gore and Associates Inc.of Flagstaff, Ariz., is bonded by a latex adhesive to the front coatedsurface of the foam substrate.

After exposure to substantial water treatment simulating use under suchenvironmental conditions, electromagnetic energy attenuation resultscomparable to Example 1 are obtained.

EXAMPLE 4

The procedure of Example 1 can be carried out using in place of thesilver, an equal amount of copper or nickel.

EXAMPLE 5

The procedure of Example 1 is carried out using a polyester polyurethanefoam substrate having a porosity of 15 ppi. In this case, however, thefoam substrate is subjected to 5 successive spray treatments or 5 passesof ink Composition A, and the treated foam substrate is then dried andcured as in Example 1.

The procedure of Example 1 is also carried out using a polyesterpolyurethane foam substrate having a porosity of 5 ppi. Here the foamsubstrate is subjected to 15 successive spray treatments or 15 passes ofink Composition A, and the treated foam substrate then dried and curedas in Example 1.

FIG. 3 is a plot illustrating the dielectric gradient profile for eachof the two foam substrates noted above, treated with the conductive inkComposition A in the manner described above. Plot X represents thedielectric gradient for the 15 ppi substrate subjected to 5 passes ofconductive ink treatment, and plot Y the dielectric gradient for the 5ppi substrate subjected to 15 passes of the conductive ink treatment.

Plots X and Y clearly show an increase in dielectric gradient or adecrease in electrical conductivity from the coated side of the foam,indicated at 0 thickness on the plot, the location of greatestelectrical conductivity, and gradually reduced conductivity as the inkpenetrates the thickness of the foam substrate, as shown by greatervalues of ohms/square for each of plots X and Y at increasingpenetrating thicknesses of the ink in each foam substrate. The term"ohms/square" as a measure of foam resistivity is defined as the bulkresistivity of the foam substrate which is expressed in ohms×cm dividedby the thickness in cm.

From the foregoing it is seen that a foam absorber is provided accordingto the invention which is effective for absorbing electromagnetic orradar energy, and has a variety of features and advantages noted above,including durability and flexibility over a broad range of frequencies,and chemical resistance.

Since various further modifications of the invention will occur to thoseskilled in the art, the invention is not to be taken as limited exceptby the scope of the appended claims.

What is claimed is:
 1. A foam material having electromagnetic energyattenuation characteristics which comprises an open cell reticulatedpolyurethane foam substrate impregnated with a conductive ink applied toa surface of the foam substrate and gradiently loading said foamsubstrate from said surface with said ink to provide a dielectricgradient, said ink comprised of an epoxy resin carrier containing aconductive material consisting essentially of a combination of carbonand a metal selected from the group consisting of silver, copper andnickel.
 2. The foam material of claim 1, said reticulated foam substratebeing a polyester polyurethane reticulated foam.
 3. The foam material ofclaim 1, said conductive material in said ink being a combination ofcarbon and silver.
 4. The foam material of claim 1, the ratio of carbonto said metal in the conductive material in said ink ranging from about0.5 to about 2 parts of carbon to about 1 part of the metal, by weight.5. The foam material of claim 3, the weight ration of carbon to silverin said conductive material being about 1.7 to about
 1. 6. The foammaterial of claim 1, wherein the conductive ink forms a coating on saidsurface of the reticulated polyurethane foam substrate and the amount ofsaid ink impregnated into the foam substrate from said surface isgradually reduced from said applied surface to a final depth ofpenetration of said ink within the body of the reticulated foamsubstrate.
 7. The foam material of claim 1, said reticulated foam havinga foam porosity ranging from about 3 to about 20 ppi (pores per inch).8. The foam material of claim 1, the thickness of the conductive inkgradient layer in said reticulated foam substrate ranging from about 0.1to about 4 inches.
 9. The foam material of claim 1, said reticulatedfoam substrate containing a fire resistant material.
 10. The foammaterial of claim 1, and including a skin material bonded to saidsurface of the reticulated foam substrate.
 11. The foam material ofclaim 10, said skin material being cloth fabric sheet or plastic sheet,adhesively or ultrasonically bonded to said surface of the reticulatedfoam substrate.
 12. A foam absorber having broadband radar attenuationcharacteristics which comprises an open cell reticulated polyurethanefoam substrate having a porosity ranging from about 3 to about 20 ppi(pores per inch) coated on the surface of said substrate with aconductive ink, said conductive ink comprised of a cured epoxy resincarrier containing a conductive material consisting essentially of acombination of carbon and silver in a weight ratio of carbon to silverof about 1.7 to about 1, the amount of said conductive ink graduallydecreasing from said coated surface to the final depth of penetration ofsaid ink within said substrate to provide a dielectric gradient.
 13. Thefoam absorber of claim 12, containing a fire retardent material.
 14. Thefoam absorber of claim 12, and including a skin material adhesivelybonded to said surface of the reticulated foam substrate, said skinmaterial being cloth, fabric sheet or plastic sheet.